The Births, Deaths, Marriages and Relationships Registration Bill "began as a simple measure to update the previous legislation and develop new digital and online channels to access births, deaths and marriages information," Ms Martin said.

"However, significant changes were made to the Bill by the select committee around gender self-identification and this occurred without adequate public consultation. This has created a fundamental legal issue."

She said self-identification clauses were added to the Bill at Select Committee, after submission on the Bill closed, meaning stakeholders may have missed an opportunity to have their say on the matter.

Changes made at the Select Committee introduced clauses allowing people to change the sex on their birth certificate via an administrative process based on self-identification.

Ms Martin said it's important for people to have their say, pointing to England and Wales, where there was a submission period of almost four months on the issue.

"There are also wider legal implications of changing to a self-identification system that need further consideration. The Crown Law Office have provided advice that the self-identification clauses would benefit from clarification," she said.

"This includes making clear the circumstances where a person's sex or gender identity might need to be determined independently of the sex shown on their birth certificate and how this should be determined."

For example, enrolment in single-sex schools would need to be looked at, as would access services at women's refuges and the criminal justice system.

The current process for amending registered sex takes place through the Family Court. On average, according to the Government, around 20 to 25 people a year use this process.

Ms Martin noted how allowing people to make changes to their birth certificate via an administrative process based on self-identification would be a "substantial change from the current Family Court process that requires evidence of medical treatment".

She said she acknowledges there are barriers preventing people from changing sex information on birth certificates, which include fees, and the costs of obtaining medical evidence or legal representation.

"The Family Court process can also be time consuming and present dignity barriers for some applicants. Many individuals find the court process adversarial and intimidating and the medical evidence requirement invasive," she said.

"I intend to initiate work to mitigate some of these barriers in the short-term, this may involve engaging with stakeholders to test mitigation proposals and how the process can be improved."

The decision to halt the law change has been welcomed by Family First New Zealand, with national director Bob McCoskrie saying it's "disturbing that some politicians want to ignore biological reality and, in the process, bring about confusion and ambiguity".

"For politicians and the state to be manipulating birth certificates in these ways is deeply concerning," he added.